The present invention relates to flexible membranes, and more particularly to a flexible diaphragm seal assembly suitable for use in extreme temperature ranges.
Flexible diaphragms used as seals, as pressure transducers, as barriers for separating differing media, and so forth, are well known. Numerous examples may be found in the technical literature, in industrial applications, and of course in nature itself. The man-made diaphragm configurations and methods for their fabrication are as varied as the applications to which they are put.
However, although there are many diaphragms designed for rigorous environments, none could be found which was suitable for certain especially demanding aerospace applications. For example, in the Shuttle Orbiter there is a need for a diaphragm seal capable of operating at temperature extremes ranging from -200.degree. F. to +600.degree. F., and capable of withstanding physical excursions imposed at these temperatures by induced motions during the Shuttle engine ignition, ascent, and External Tank and Orbiter separation. In one instance, these motions require a diaphragm capable of deflecting 0.5 inch in all directions while subjected to -150.degree. F. .+-.10.degree. F., and while containing an inert gaseous purge. The leakage is not to exceed 10.4 SCFM @ 3.26 PSI, 18.8 SCFM @ -2.76 PSI. After external tank. Orbiter separation, the diaphragm then has to provide a seal between the aft fuselage area and outer space. The gaseous purge requirement (as with an inert gas to prevent explosive risks) means that the diaphragm seal must be capable of maintaining a positive pressure during such maneuvers. After separation in outer space, these pressure conditions then reverse. No known material or combination of materials could be found which was capable of operating in these extreme conditions.
Finding the prior art wanting, various configurations, such as silicon rubber coated aramid, teflon, aluminum foil, and glass fabric composite sandwich diaphragms, were constructed and tested in attempts to meet these stringent requirements. They all failed during cryogenic flexing tests.
That the literature fails to disclose or suggest configurations suitable for such extreme conditions may be seen from the following examples. U.S. Pat. No. 2,929,655 (Dwyer, issued Jan. 12, 1960), for instance, discloses a diaphragm made of a fibrous base layer (for example, glass cloth) having flexible metal foil on one or both faces thereof. It is intended for above normal operating temperatures (not cryogenic)--the metal being described, inter alia, as having to have a melting point higher than that to which it would be subjected in use. Due to brittleness of metal at low temperatures it is doubtful whether such a diaphragm would be suitable for low temperature extremes.
U.S. Pat. No. 3,026,909 (Boteler, issued Mar. 27, 1962) discloses a stud-containing diaphragm for use in diaphragm valves. The diaphragm is reinforced to provide high pull-out strength for the stud, and is cured by conventional compression molding at an elevated temperature in a hydraulic press, as distinguished from the vacuum autoclave pressure molding taught further herein by the present invention.
U.S. Pat. No. 4,022,114 (Hansen, III et al., issued May 10, 1977) is directed to diaphragms for use in regulators for handling refrigerant media passing from an evaporator to a compressor. With regard to the disclosed diaphragms, it teaches that "Total and complete bonding . . . is never possible" (column 1, lines 21-23), and that "voids, however small, between the bonded surfaces are inevitable." (Column 1, lines 23-24.) In fact, it states that ". . . no degree of care can prevent the existence of voids or weakened places in the bonded interface of the laminates." (Column 1, lines 34-37.) Several structures are therefore disclosed, depending upon the intended application. These include a fiberglass reinforcing layer bonded to elastomer sealing layers, which may be silicone rubber. It seeks to solve the problem of imperfect bonding by perforating the elastomer to vent any gas trapped in voids between the layers. Sometimes the perforations are only partial. In some embodiments the bonding of the elastomer to the fabric is intentionally only partial. Some have fabric or elastomer on one side (surface) only.
A two-sided silicone rubber coated glass fabric configuration, produced by liquid silicone rubber processing, such as supported extrusions, is disclosed in Elastomerics, 112, Feb. 1980, pp. 17-20 (W. R. Hays). Unfortunately, tests of this structure for use as an aerospace diaphragm (using aramide fabric reinforcement rather than glass fabric) found that the diaphragm cracked and leaked in several places during initial chill down to -150.degree. F. .+-.10.degree. F., prior to the initiation of testing. Retesting (of the same configuration and material, but with material from a different supplier) resulted in the same failure. As presently understood, this failure is believed to have resulted from the necessity to thin the liquid rubber so that it could impregnate into the weave or knit of the fabric. Such thinning lowers the molecular weight consistency, thereby reducing the rubber film strength.
A substantial need therefore remains for a flexible diaphragm suitable for extreme temperature usage, and particularly one which can meet the rigorous aerospace applications described above. Ideally, such a diaphragm construction, and its method of fabrication, will be not only durable (and thus suitable for multiple aerospace mission use), but will also be uncomplicated, versatile, and relatively inexpensive to manufacture. In addition, it should ideally be suitable for use in a great variety of other applications, such as refrigeration seals, autoclaves, storage lockers, and other sealing applications subjected to extreme temperature differentials.